Hitherto, in the constant speed drive unit or cruise control of the stepper motor type, the characteristic of the throttle valve opening speed with respect to the driving speed of the automobile (hereinafter referred to as "gain characteristic") is almost flat; in other words, a constant gain characteristic is given to the conventional constant speed unit.
Driving performance as well as acceleration performance varies depending on the type of automobile. For example, a sports car is better in driving performance than a commercial vehicle or a passenger car, and acceleration performance of a sports car is also higher. Accordingly, higher responsiveness is desired in the constant speed drive unit of the sports car, and the gain characteristic thereof is desired to be the approximately linear in a relation to drive speed thereof.
On the other hand, in commercial vehicle, high responsiveness is not required as it is with a sports car in view of the high loading capacity, but a flat gain characteristic is desired for every driving speed thereof.
In a passenger car, a linear characteristic similar to that of a sports car is required up to an intermediate speed region, while a saturation or lowering characteristic is desired in the high speed region in view of safety concerns.
Since a characteristic, whose gain characteristic is almost flat, is given to the conventional constant speed drive unit as mentioned above, a problem exists in that it is impossible to obtain a desirable gain characteristic flexibly corresponding to the type of automobile.
The present invention was made to solve the above problem and has an object of providing a constant speed drive unit in which the pulse motor speed is flexibly controlled corresponding to the gain characteristic, whereby controllability and stability are both improved.